/**
 * @file
 * DNS - host name to IP address resolver.
 *
 */

/**

 * This file implements a DNS host name to IP address resolver.

 * Port to lwIP from uIP
 * by Jim Pettinato April 2007

 * uIP version Copyright (c) 2002-2003, Adam Dunkels.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior
 *    written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * DNS.C
 *
 * The lwIP DNS resolver functions are used to lookup a host name and
 * map it to a numerical IP address. It maintains a list of resolved
 * hostnames that can be queried with the dns_lookup() function.
 * New hostnames can be resolved using the dns_query() function.
 *
 * The lwIP version of the resolver also adds a non-blocking version of
 * gethostbyname() that will work with a raw API application. This function
 * checks for an IP address string first and converts it if it is valid.
 * gethostbyname() then does a dns_lookup() to see if the name is
 * already in the table. If so, the IP is returned. If not, a query is
 * issued and the function returns with a ERR_INPROGRESS status. The app
 * using the dns client must then go into a waiting state.
 *
 * Once a hostname has been resolved (or found to be non-existent),
 * the resolver code calls a specified callback function (which
 * must be implemented by the module that uses the resolver).
 */

/*-----------------------------------------------------------------------------
 * RFC 1035 - Domain names - implementation and specification
 * RFC 2181 - Clarifications to the DNS Specification
 *----------------------------------------------------------------------------*/

/** @todo: define good default values (rfc compliance) */
/** @todo: improve answer parsing, more checkings... */
/** @todo: check RFC1035 - 7.3. Processing responses */

/*-----------------------------------------------------------------------------
 * Includes
 *----------------------------------------------------------------------------*/

#include "lwip/opt.h"

#if LWIP_DNS /* don't build if not configured for use in lwipopts.h */

#include "lwip/udp.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/dns.h"

#include <string.h>

/** DNS server IP address */
#ifndef DNS_SERVER_ADDRESS
#define DNS_SERVER_ADDRESS(ipaddr) (ip4_addr_set_u32(ipaddr, ipaddr_addr("208.67.222.222"))) /* resolver1.opendns.com  \
	                                                                                          */
#endif

/** DNS server port address */
#ifndef DNS_SERVER_PORT
#define DNS_SERVER_PORT 53
#endif

/** DNS maximum number of retries when asking for a name, before "timeout". */
#ifndef DNS_MAX_RETRIES
#define DNS_MAX_RETRIES 4
#endif

/** DNS resource record max. TTL (one week as default) */
#ifndef DNS_MAX_TTL
#define DNS_MAX_TTL 604800
#endif

/* DNS protocol flags */
#define DNS_FLAG1_RESPONSE 0x80
#define DNS_FLAG1_OPCODE_STATUS 0x10
#define DNS_FLAG1_OPCODE_INVERSE 0x08
#define DNS_FLAG1_OPCODE_STANDARD 0x00
#define DNS_FLAG1_AUTHORATIVE 0x04
#define DNS_FLAG1_TRUNC 0x02
#define DNS_FLAG1_RD 0x01
#define DNS_FLAG2_RA 0x80
#define DNS_FLAG2_ERR_MASK 0x0f
#define DNS_FLAG2_ERR_NONE 0x00
#define DNS_FLAG2_ERR_NAME 0x03

/* DNS protocol states */
#define DNS_STATE_UNUSED 0
#define DNS_STATE_NEW 1
#define DNS_STATE_ASKING 2
#define DNS_STATE_DONE 3

#ifdef PACK_STRUCT_USE_INCLUDES
#include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS message header */
struct dns_hdr {
	PACK_STRUCT_FIELD(u16_t id);
	PACK_STRUCT_FIELD(u8_t flags1);
	PACK_STRUCT_FIELD(u8_t flags2);
	PACK_STRUCT_FIELD(u16_t numquestions);
	PACK_STRUCT_FIELD(u16_t numanswers);
	PACK_STRUCT_FIELD(u16_t numauthrr);
	PACK_STRUCT_FIELD(u16_t numextrarr);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
#include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_HDR 12

/** DNS query message structure.
    No packing needed: only used locally on the stack. */
struct dns_query {
	/* DNS query record starts with either a domain name or a pointer
	   to a name already present somewhere in the packet. */
	u16_t type;
	u16_t cls;
};
#define SIZEOF_DNS_QUERY 4

/** DNS answer message structure.
    No packing needed: only used locally on the stack. */
struct dns_answer {
	/* DNS answer record starts with either a domain name or a pointer
	   to a name already present somewhere in the packet. */
	u16_t type;
	u16_t cls;
	u32_t ttl;
	u16_t len;
};
#define SIZEOF_DNS_ANSWER 10

/** DNS table entry */
struct dns_table_entry {
	u8_t      state;
	u8_t      numdns;
	u8_t      tmr;
	u8_t      retries;
	u8_t      seqno;
	u8_t      err;
	u32_t     ttl;
	char      name[DNS_MAX_NAME_LENGTH];
	ip_addr_t ipaddr;
	/* pointer to callback on DNS query done */
	dns_found_callback found;
	void *             arg;
};

#if DNS_LOCAL_HOSTLIST

#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/** Local host-list. For hostnames in this list, no
 *  external name resolution is performed */
static struct local_hostlist_entry *local_hostlist_dynamic;
#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

/** Defining this allows the local_hostlist_static to be placed in a different
 * linker section (e.g. FLASH) */
#ifndef DNS_LOCAL_HOSTLIST_STORAGE_PRE
#define DNS_LOCAL_HOSTLIST_STORAGE_PRE static
#endif /* DNS_LOCAL_HOSTLIST_STORAGE_PRE */
/** Defining this allows the local_hostlist_static to be placed in a different
 * linker section (e.g. FLASH) */
#ifndef DNS_LOCAL_HOSTLIST_STORAGE_POST
#define DNS_LOCAL_HOSTLIST_STORAGE_POST
#endif /* DNS_LOCAL_HOSTLIST_STORAGE_POST */
DNS_LOCAL_HOSTLIST_STORAGE_PRE struct local_hostlist_entry local_hostlist_static[] DNS_LOCAL_HOSTLIST_STORAGE_POST
    = DNS_LOCAL_HOSTLIST_INIT;

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

static void dns_init_local();
#endif /* DNS_LOCAL_HOSTLIST */

/* forward declarations */
static void dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, ip_addr_t *addr, u16_t port);
static void dns_check_entries(void);

/*-----------------------------------------------------------------------------
 * Globales
 *----------------------------------------------------------------------------*/

/* DNS variables */
static struct udp_pcb *       dns_pcb;
static u8_t                   dns_seqno;
static struct dns_table_entry dns_table[DNS_TABLE_SIZE];
static ip_addr_t              dns_servers[DNS_MAX_SERVERS];
/** Contiguous buffer for processing responses */
static u8_t  dns_payload_buffer[LWIP_MEM_ALIGN_BUFFER(DNS_MSG_SIZE)];
static u8_t *dns_payload;

/**
 * Initialize the resolver: set up the UDP pcb and configure the default server
 * (DNS_SERVER_ADDRESS).
 */
void dns_init()
{
	ip_addr_t dnsserver;

	dns_payload = (u8_t *)LWIP_MEM_ALIGN(dns_payload_buffer);

	/* initialize default DNS server address */
	DNS_SERVER_ADDRESS(&dnsserver);

	LWIP_DEBUGF(DNS_DEBUG, ("dns_init: initializing\n"));

	/* if dns client not yet initialized... */
	if (dns_pcb == NULL) {
		dns_pcb = udp_new();

		if (dns_pcb != NULL) {
			/* initialize DNS table not needed (initialized to zero since it is a
			 * global variable) */
			LWIP_ASSERT("For implicit initialization to work, DNS_STATE_UNUSED needs to be 0", DNS_STATE_UNUSED == 0);

			/* initialize DNS client */
			udp_bind(dns_pcb, IP_ADDR_ANY, 0);
			udp_recv(dns_pcb, dns_recv, NULL);

			/* initialize default DNS primary server */
			dns_setserver(0, &dnsserver);
		}
	}
#if DNS_LOCAL_HOSTLIST
	dns_init_local();
#endif
}

/**
 * Initialize one of the DNS servers.
 *
 * @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS
 * @param dnsserver IP address of the DNS server to set
 */
void dns_setserver(u8_t numdns, ip_addr_t *dnsserver)
{
	if ((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) && (dnsserver != NULL) && !ip_addr_isany(dnsserver)) {
		dns_servers[numdns] = (*dnsserver);
	}
}

/**
 * Obtain one of the currently configured DNS server.
 *
 * @param numdns the index of the DNS server
 * @return IP address of the indexed DNS server or "ip_addr_any" if the DNS
 *         server has not been configured.
 */
ip_addr_t dns_getserver(u8_t numdns)
{
	if (numdns < DNS_MAX_SERVERS) {
		return dns_servers[numdns];
	} else {
		return *IP_ADDR_ANY;
	}
}

/**
 * The DNS resolver client timer - handle retries and timeouts and should
 * be called every DNS_TMR_INTERVAL milliseconds (every second by default).
 */
void dns_tmr(void)
{
	if (dns_pcb != NULL) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_tmr: dns_check_entries\n"));
		dns_check_entries();
	}
}

#if DNS_LOCAL_HOSTLIST
static void dns_init_local()
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT)
	int                          i;
	struct local_hostlist_entry *entry;
	/* Dynamic: copy entries from DNS_LOCAL_HOSTLIST_INIT to list */
	struct local_hostlist_entry local_hostlist_init[] = DNS_LOCAL_HOSTLIST_INIT;
	size_t                      namelen;
	for (i = 0; i < sizeof(local_hostlist_init) / sizeof(struct local_hostlist_entry); i++) {
		struct local_hostlist_entry *init_entry = &local_hostlist_init[i];
		LWIP_ASSERT("invalid host name (NULL)", init_entry->name != NULL);
		namelen = strlen(init_entry->name);
		LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
		entry = (struct local_hostlist_entry *)memp_malloc(MEMP_LOCALHOSTLIST);
		LWIP_ASSERT("mem-error in dns_init_local", entry != NULL);
		if (entry != NULL) {
			entry->name = (char *)entry + sizeof(struct local_hostlist_entry);
			MEMCPY((char *)entry->name, init_entry->name, namelen);
			((char *)entry->name)[namelen] = 0;
			entry->addr                    = init_entry->addr;
			entry->next                    = local_hostlist_dynamic;
			local_hostlist_dynamic         = entry;
		}
	}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT) */
}

/**
 * Scans the local host-list for a hostname.
 *
 * @param hostname Hostname to look for in the local host-list
 * @return The first IP address for the hostname in the local host-list or
 *         IPADDR_NONE if not found.
 */
static u32_t dns_lookup_local(const char *hostname)
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
	struct local_hostlist_entry *entry = local_hostlist_dynamic;
	while (entry != NULL) {
		if (strcmp(entry->name, hostname) == 0) {
			return ip4_addr_get_u32(&entry->addr);
		}
		entry = entry->next;
	}
#else  /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
	int i;
	for (i = 0; i < sizeof(local_hostlist_static) / sizeof(struct local_hostlist_entry); i++) {
		if (strcmp(local_hostlist_static[i].name, hostname) == 0) {
			return ip4_addr_get_u32(&local_hostlist_static[i].addr);
		}
	}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
	return IPADDR_NONE;
}

#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/** Remove all entries from the local host-list for a specific hostname
 * and/or IP addess
 *
 * @param hostname hostname for which entries shall be removed from the local
 *                 host-list
 * @param addr address for which entries shall be removed from the local host-list
 * @return the number of removed entries
 */
int dns_local_removehost(const char *hostname, const ip_addr_t *addr)
{
	int                          removed    = 0;
	struct local_hostlist_entry *entry      = local_hostlist_dynamic;
	struct local_hostlist_entry *last_entry = NULL;
	while (entry != NULL) {
		if (((hostname == NULL) || !strcmp(entry->name, hostname))
		    && ((addr == NULL) || ip_addr_cmp(&entry->addr, addr))) {
			struct local_hostlist_entry *free_entry;
			if (last_entry != NULL) {
				last_entry->next = entry->next;
			} else {
				local_hostlist_dynamic = entry->next;
			}
			free_entry = entry;
			entry      = entry->next;
			memp_free(MEMP_LOCALHOSTLIST, free_entry);
			removed++;
		} else {
			last_entry = entry;
			entry      = entry->next;
		}
	}
	return removed;
}

/**
 * Add a hostname/IP address pair to the local host-list.
 * Duplicates are not checked.
 *
 * @param hostname hostname of the new entry
 * @param addr IP address of the new entry
 * @return ERR_OK if succeeded or ERR_MEM on memory error
 */
err_t dns_local_addhost(const char *hostname, const ip_addr_t *addr)
{
	struct local_hostlist_entry *entry;
	size_t                       namelen;
	LWIP_ASSERT("invalid host name (NULL)", hostname != NULL);
	namelen = strlen(hostname);
	LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
	entry = (struct local_hostlist_entry *)memp_malloc(MEMP_LOCALHOSTLIST);
	if (entry == NULL) {
		return ERR_MEM;
	}
	entry->name = (char *)entry + sizeof(struct local_hostlist_entry);
	MEMCPY((char *)entry->name, hostname, namelen);
	((char *)entry->name)[namelen] = 0;
	ip_addr_copy(entry->addr, *addr);
	entry->next            = local_hostlist_dynamic;
	local_hostlist_dynamic = entry;
	return ERR_OK;
}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC*/
#endif /* DNS_LOCAL_HOSTLIST */

/**
 * Look up a hostname in the array of known hostnames.
 *
 * @note This function only looks in the internal array of known
 * hostnames, it does not send out a query for the hostname if none
 * was found. The function dns_enqueue() can be used to send a query
 * for a hostname.
 *
 * @param name the hostname to look up
 * @return the hostname's IP address, as u32_t (instead of ip_addr_t to
 *         better check for failure: != IPADDR_NONE) or IPADDR_NONE if the hostname
 *         was not found in the cached dns_table.
 */
static u32_t dns_lookup(const char *name)
{
	u8_t i;
#if DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN)
	u32_t addr;
#endif /* DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN) */
#if DNS_LOCAL_HOSTLIST
	if ((addr = dns_lookup_local(name)) != IPADDR_NONE) {
		return addr;
	}
#endif /* DNS_LOCAL_HOSTLIST */
#ifdef DNS_LOOKUP_LOCAL_EXTERN
	if ((addr = DNS_LOOKUP_LOCAL_EXTERN(name)) != IPADDR_NONE) {
		return addr;
	}
#endif /* DNS_LOOKUP_LOCAL_EXTERN */

	/* Walk through name list, return entry if found. If not, return NULL. */
	for (i = 0; i < DNS_TABLE_SIZE; ++i) {
		if ((dns_table[i].state == DNS_STATE_DONE) && (strcmp(name, dns_table[i].name) == 0)) {
			LWIP_DEBUGF(DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
			ip_addr_debug_print(DNS_DEBUG, &(dns_table[i].ipaddr));
			LWIP_DEBUGF(DNS_DEBUG, ("\n"));
			return ip4_addr_get_u32(&dns_table[i].ipaddr);
		}
	}

	return IPADDR_NONE;
}

#if DNS_DOES_NAME_CHECK
/**
 * Compare the "dotted" name "query" with the encoded name "response"
 * to make sure an answer from the DNS server matches the current dns_table
 * entry (otherwise, answers might arrive late for hostname not on the list
 * any more).
 *
 * @param query hostname (not encoded) from the dns_table
 * @param response encoded hostname in the DNS response
 * @return 0: names equal; 1: names differ
 */
static u8_t dns_compare_name(unsigned char *query, unsigned char *response)
{
	unsigned char n;

	do {
		n = *response++;
		/** @see RFC 1035 - 4.1.4. Message compression */
		if ((n & 0xc0) == 0xc0) {
			/* Compressed name */
			break;
		} else {
			/* Not compressed name */
			while (n > 0) {
				if ((*query) != (*response)) {
					return 1;
				}
				++response;
				++query;
				--n;
			};
			++query;
		}
	} while (*response != 0);

	return 0;
}
#endif /* DNS_DOES_NAME_CHECK */

/**
 * Walk through a compact encoded DNS name and return the end of the name.
 *
 * @param query encoded DNS name in the DNS server response
 * @return end of the name
 */
static unsigned char *dns_parse_name(unsigned char *query)
{
	unsigned char n;

	do {
		n = *query++;
		/** @see RFC 1035 - 4.1.4. Message compression */
		if ((n & 0xc0) == 0xc0) {
			/* Compressed name */
			break;
		} else {
			/* Not compressed name */
			while (n > 0) {
				++query;
				--n;
			};
		}
	} while (*query != 0);

	return query + 1;
}

/**
 * Send a DNS query packet.
 *
 * @param numdns index of the DNS server in the dns_servers table
 * @param name hostname to query
 * @param id index of the hostname in dns_table, used as transaction ID in the
 *        DNS query packet
 * @return ERR_OK if packet is sent; an err_t indicating the problem otherwise
 */
static err_t dns_send(u8_t numdns, const char *name, u8_t id)
{
	err_t            err;
	struct dns_hdr * hdr;
	struct dns_query qry;
	struct pbuf *    p;
	char *           query, *nptr;
	const char *     pHostname;
	u8_t             n;

	LWIP_DEBUGF(DNS_DEBUG, ("dns_send: dns_servers[%" U16_F "] \"%s\": request\n", (u16_t)(numdns), name));
	LWIP_ASSERT("dns server out of array", numdns < DNS_MAX_SERVERS);
	LWIP_ASSERT("dns server has no IP address set", !ip_addr_isany(&dns_servers[numdns]));

	/* if here, we have either a new query or a retry on a previous query to process */
	p = pbuf_alloc(PBUF_TRANSPORT, SIZEOF_DNS_HDR + DNS_MAX_NAME_LENGTH + SIZEOF_DNS_QUERY, PBUF_RAM);
	if (p != NULL) {
		LWIP_ASSERT("pbuf must be in one piece", p->next == NULL);
		/* fill dns header */
		hdr = (struct dns_hdr *)p->payload;
		memset(hdr, 0, SIZEOF_DNS_HDR);
		hdr->id           = htons(id);
		hdr->flags1       = DNS_FLAG1_RD;
		hdr->numquestions = PP_HTONS(1);
		query             = (char *)hdr + SIZEOF_DNS_HDR;
		pHostname         = name;
		--pHostname;

		/* convert hostname into suitable query format. */
		do {
			++pHostname;
			nptr = query;
			++query;
			for (n = 0; *pHostname != '.' && *pHostname != 0; ++pHostname) {
				*query = *pHostname;
				++query;
				++n;
			}
			*nptr = n;
		} while (*pHostname != 0);
		*query++ = '\0';

		/* fill dns query */
		qry.type = PP_HTONS(DNS_RRTYPE_A);
		qry.cls  = PP_HTONS(DNS_RRCLASS_IN);
		SMEMCPY(query, &qry, SIZEOF_DNS_QUERY);

		/* resize pbuf to the exact dns query */
		pbuf_realloc(p, (u16_t)((query + SIZEOF_DNS_QUERY) - ((char *)(p->payload))));

		/* connect to the server for faster receiving */
		udp_connect(dns_pcb, &dns_servers[numdns], DNS_SERVER_PORT);
		/* send dns packet */
		err = udp_sendto(dns_pcb, p, &dns_servers[numdns], DNS_SERVER_PORT);

		/* free pbuf */
		pbuf_free(p);
	} else {
		err = ERR_MEM;
	}

	return err;
}

/**
 * dns_check_entry() - see if pEntry has not yet been queried and, if so, sends out a query.
 * Check an entry in the dns_table:
 * - send out query for new entries
 * - retry old pending entries on timeout (also with different servers)
 * - remove completed entries from the table if their TTL has expired
 *
 * @param i index of the dns_table entry to check
 */
static void dns_check_entry(u8_t i)
{
	err_t                   err;
	struct dns_table_entry *pEntry = &dns_table[i];

	LWIP_ASSERT("array index out of bounds", i < DNS_TABLE_SIZE);

	switch (pEntry->state) {

	case DNS_STATE_NEW: {
		/* initialize new entry */
		pEntry->state   = DNS_STATE_ASKING;
		pEntry->numdns  = 0;
		pEntry->tmr     = 1;
		pEntry->retries = 0;

		/* send DNS packet for this entry */
		err = dns_send(pEntry->numdns, pEntry->name, i);
		if (err != ERR_OK) {
			LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING, ("dns_send returned error: %s\n", lwip_strerr(err)));
		}
		break;
	}

	case DNS_STATE_ASKING: {
		if (--pEntry->tmr == 0) {
			if (++pEntry->retries == DNS_MAX_RETRIES) {
				if ((pEntry->numdns + 1 < DNS_MAX_SERVERS) && !ip_addr_isany(&dns_servers[pEntry->numdns + 1])) {
					/* change of server */
					pEntry->numdns++;
					pEntry->tmr     = 1;
					pEntry->retries = 0;
					break;
				} else {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": timeout\n", pEntry->name));
					/* call specified callback function if provided */
					if (pEntry->found)
						(*pEntry->found)(pEntry->name, NULL, pEntry->arg);
					/* flush this entry */
					pEntry->state = DNS_STATE_UNUSED;
					pEntry->found = NULL;
					break;
				}
			}

			/* wait longer for the next retry */
			pEntry->tmr = pEntry->retries;

			/* send DNS packet for this entry */
			err = dns_send(pEntry->numdns, pEntry->name, i);
			if (err != ERR_OK) {
				LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING, ("dns_send returned error: %s\n", lwip_strerr(err)));
			}
		}
		break;
	}

	case DNS_STATE_DONE: {
		/* if the time to live is nul */
		if (--pEntry->ttl == 0) {
			LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
			/* flush this entry */
			pEntry->state = DNS_STATE_UNUSED;
			pEntry->found = NULL;
		}
		break;
	}
	case DNS_STATE_UNUSED:
		/* nothing to do */
		break;
	default:
		LWIP_ASSERT("unknown dns_table entry state:", 0);
		break;
	}
}

/**
 * Call dns_check_entry for each entry in dns_table - check all entries.
 */
static void dns_check_entries(void)
{
	u8_t i;

	for (i = 0; i < DNS_TABLE_SIZE; ++i) {
		dns_check_entry(i);
	}
}

/**
 * Receive input function for DNS response packets arriving for the dns UDP pcb.
 *
 * @params see udp.h
 */
static void dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, ip_addr_t *addr, u16_t port)
{
	u16_t                   i;
	char *                  pHostname;
	struct dns_hdr *        hdr;
	struct dns_answer       ans;
	struct dns_table_entry *pEntry;
	u16_t                   nquestions, nanswers;

	LWIP_UNUSED_ARG(arg);
	LWIP_UNUSED_ARG(pcb);
	LWIP_UNUSED_ARG(addr);
	LWIP_UNUSED_ARG(port);

	/* is the dns message too big ? */
	if (p->tot_len > DNS_MSG_SIZE) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too big\n"));
		/* free pbuf and return */
		goto memerr;
	}

	/* is the dns message big enough ? */
	if (p->tot_len < (SIZEOF_DNS_HDR + SIZEOF_DNS_QUERY + SIZEOF_DNS_ANSWER)) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
		/* free pbuf and return */
		goto memerr;
	}

	/* copy dns payload inside static buffer for processing */
	if (pbuf_copy_partial(p, dns_payload, p->tot_len, 0) == p->tot_len) {
		/* The ID in the DNS header should be our entry into the name table. */
		hdr = (struct dns_hdr *)dns_payload;
		i   = htons(hdr->id);
		if (i < DNS_TABLE_SIZE) {
			pEntry = &dns_table[i];
			if (pEntry->state == DNS_STATE_ASKING) {
				/* This entry is now completed. */
				pEntry->state = DNS_STATE_DONE;
				pEntry->err   = hdr->flags2 & DNS_FLAG2_ERR_MASK;

				/* We only care about the question(s) and the answers. The authrr
				   and the extrarr are simply discarded. */
				nquestions = htons(hdr->numquestions);
				nanswers   = htons(hdr->numanswers);

				/* Check for error. If so, call callback to inform. */
				if (((hdr->flags1 & DNS_FLAG1_RESPONSE) == 0) || (pEntry->err != 0) || (nquestions != 1)) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in flags\n", pEntry->name));
					/* call callback to indicate error, clean up memory and return */
					goto responseerr;
				}

#if DNS_DOES_NAME_CHECK
				/* Check if the name in the "question" part match with the name in the entry. */
				if (dns_compare_name((unsigned char *)(pEntry->name), (unsigned char *)dns_payload + SIZEOF_DNS_HDR)
				    != 0) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", pEntry->name));
					/* call callback to indicate error, clean up memory and return */
					goto responseerr;
				}
#endif /* DNS_DOES_NAME_CHECK */

				/* Skip the name in the "question" part */
				pHostname = (char *)dns_parse_name((unsigned char *)dns_payload + SIZEOF_DNS_HDR) + SIZEOF_DNS_QUERY;

				while (nanswers > 0) {
					/* skip answer resource record's host name */
					pHostname = (char *)dns_parse_name((unsigned char *)pHostname);

					/* Check for IP address type and Internet class. Others are discarded. */
					SMEMCPY(&ans, pHostname, SIZEOF_DNS_ANSWER);
					if ((ans.type == PP_HTONS(DNS_RRTYPE_A)) && (ans.cls == PP_HTONS(DNS_RRCLASS_IN))
					    && (ans.len == PP_HTONS(sizeof(ip_addr_t)))) {
						/* read the answer resource record's TTL, and maximize it if needed */
						pEntry->ttl = ntohl(ans.ttl);
						if (pEntry->ttl > DNS_MAX_TTL) {
							pEntry->ttl = DNS_MAX_TTL;
						}
						/* read the IP address after answer resource record's header */
						SMEMCPY(&(pEntry->ipaddr), (pHostname + SIZEOF_DNS_ANSWER), sizeof(ip_addr_t));
						LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response = ", pEntry->name));
						ip_addr_debug_print(DNS_DEBUG, (&(pEntry->ipaddr)));
						LWIP_DEBUGF(DNS_DEBUG, ("\n"));
						/* call specified callback function if provided */
						if (pEntry->found) {
							(*pEntry->found)(pEntry->name, &pEntry->ipaddr, pEntry->arg);
						}
						/* deallocate memory and return */
						goto memerr;
					} else {
						pHostname = pHostname + SIZEOF_DNS_ANSWER + htons(ans.len);
					}
					--nanswers;
				}
				LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in response\n", pEntry->name));
				/* call callback to indicate error, clean up memory and return */
				goto responseerr;
			}
		}
	}

	/* deallocate memory and return */
	goto memerr;

responseerr:
	/* ERROR: call specified callback function with NULL as name to indicate an error */
	if (pEntry->found) {
		(*pEntry->found)(pEntry->name, NULL, pEntry->arg);
	}
	/* flush this entry */
	pEntry->state = DNS_STATE_UNUSED;
	pEntry->found = NULL;

memerr:
	/* free pbuf */
	pbuf_free(p);
	return;
}

/**
 * Queues a new hostname to resolve and sends out a DNS query for that hostname
 *
 * @param name the hostname that is to be queried
 * @param found a callback founction to be called on success, failure or timeout
 * @param callback_arg argument to pass to the callback function
 * @return @return a err_t return code.
 */
static err_t dns_enqueue(const char *name, dns_found_callback found, void *callback_arg)
{
	u8_t                    i;
	u8_t                    lseq, lseqi;
	struct dns_table_entry *pEntry = NULL;
	size_t                  namelen;

	/* search an unused entry, or the oldest one */
	lseq = lseqi = 0;
	for (i = 0; i < DNS_TABLE_SIZE; ++i) {
		pEntry = &dns_table[i];
		/* is it an unused entry ? */
		if (pEntry->state == DNS_STATE_UNUSED)
			break;

		/* check if this is the oldest completed entry */
		if (pEntry->state == DNS_STATE_DONE) {
			if ((dns_seqno - pEntry->seqno) > lseq) {
				lseq  = dns_seqno - pEntry->seqno;
				lseqi = i;
			}
		}
	}

	/* if we don't have found an unused entry, use the oldest completed one */
	if (i == DNS_TABLE_SIZE) {
		if ((lseqi >= DNS_TABLE_SIZE) || (dns_table[lseqi].state != DNS_STATE_DONE)) {
			/* no entry can't be used now, table is full */
			LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": DNS entries table is full\n", name));
			return ERR_MEM;
		} else {
			/* use the oldest completed one */
			i      = lseqi;
			pEntry = &dns_table[i];
		}
	}

	/* use this entry */
	LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": use DNS entry %" U16_F "\n", name, (u16_t)(i)));

	/* fill the entry */
	pEntry->state = DNS_STATE_NEW;
	pEntry->seqno = dns_seqno++;
	pEntry->found = found;
	pEntry->arg   = callback_arg;
	namelen       = LWIP_MIN(strlen(name), DNS_MAX_NAME_LENGTH - 1);
	MEMCPY(pEntry->name, name, namelen);
	pEntry->name[namelen] = 0;

	/* force to send query without waiting timer */
	dns_check_entry(i);

	/* dns query is enqueued */
	return ERR_INPROGRESS;
}

/**
 * Resolve a hostname (string) into an IP address.
 * NON-BLOCKING callback version for use with raw API!!!
 *
 * Returns immediately with one of err_t return codes:
 * - ERR_OK if hostname is a valid IP address string or the host
 *   name is already in the local names table.
 * - ERR_INPROGRESS enqueue a request to be sent to the DNS server
 *   for resolution if no errors are present.
 * - ERR_ARG: dns client not initialized or invalid hostname
 *
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a ip_addr_t where to store the address if it is already
 *             cached in the dns_table (only valid if ERR_OK is returned!)
 * @param found a callback function to be called on success, failure or timeout (only if
 *              ERR_INPROGRESS is returned!)
 * @param callback_arg argument to pass to the callback function
 * @return a err_t return code.
 */
err_t dns_gethostbyname(const char *hostname, ip_addr_t *addr, dns_found_callback found, void *callback_arg)
{
	u32_t ipaddr;
	/* not initialized or no valid server yet, or invalid addr pointer
	 * or invalid hostname or invalid hostname length */
	if ((dns_pcb == NULL) || (addr == NULL) || (!hostname) || (!hostname[0])
	    || (strlen(hostname) >= DNS_MAX_NAME_LENGTH)) {
		return ERR_ARG;
	}

#if LWIP_HAVE_LOOPIF
	if (strcmp(hostname, "localhost") == 0) {
		ip_addr_set_loopback(addr);
		return ERR_OK;
	}
#endif /* LWIP_HAVE_LOOPIF */

	/* host name already in octet notation? set ip addr and return ERR_OK */
	ipaddr = ipaddr_addr(hostname);
	if (ipaddr == IPADDR_NONE) {
		/* already have this address cached? */
		ipaddr = dns_lookup(hostname);
	}
	if (ipaddr != IPADDR_NONE) {
		ip4_addr_set_u32(addr, ipaddr);
		return ERR_OK;
	}

	/* queue query with specified callback */
	return dns_enqueue(hostname, found, callback_arg);
}

#endif /* LWIP_DNS */
